1. Technical Field
The present invention relates to a communication system having a plurality of communication nodes connected to a communication bus.
2. Related Art
Known communication protocols, which are implemented in in-vehicle communication systems having a plurality of communication nodes connected to a communication bus, include:                CSMA/NBA such as Controller Area Network (CAN) compliant with ISO11898-1; and        Time Division Multiplex (TDM) based on fixed-length timeslots such as ISO11898-4 and FlexRay® (see e.g. Japanese Unexamined Patent Application Publication No. 2008-049976).        
The CAN protocol arbitrates collision of communications by using identifiers (IDs). However, an increasing number of kinds of data transmitted onto a communication bus increase the arbitration frequency. Therefore, the problem encountered in such a case is that data having an identifier (ID) of a lower priority such that it loses the arbitration with a higher probability becomes more difficult to be transmitted (or sent-out).
On the other hand, since TDM based on fixed-length timeslots such as FlexRay®, transmits one piece of data in one timeslot and its timeslots are of a fixed length, the length of timeslots must be adjusted to the longest data length in advance. Therefore, the problem with such a protocol is that in case longer length data and shorter length data coexist, usage efficiency of a communication bus may be decreased.